This invention relates to motor controllers. In particular, this invention relates to a means of controlling polyphase induction motors, including those designed for starting by being placed directly across a line and those used with a reduced-voltage controller.
Control of polyphase induction motors may include such other features as action in response to detection of the loss of one or more phases in the power supplied to the machine, detection of a load jam in a motor that has previously been brought to speed, and detection of an underload or loss of load in a motor that has been at speed. The typical polyphase induction motor that is designed for starting across an AC line draws a maximum current of the order six to eight times the rated full-load current during a limited starting interval. The starting contactors for the motor are not normally designed to handle the short-circuit current that can be delivered to a short circuit across the power lines, so this protection must be supplied by fuses or circuit breakers that are also placed in the circuit. The full-load current, when carried continuously by the motor, represents a tolerable level of heating. Six or eight times that full-load current will normally exceed the steady-state capability of the motor and it is therefore necessary to limit both the time that such a current is drawn and the number of intervals during a given period of time in which such a current is drawn. In other words, the motor can carry an overload by a factor of six to eight for a limited period of time, but beyond that time the motor must be disconnected from the power supply to protect it. If the motor has been overloaded, either in starting for too long a period or in carrying too high a current in the steady-state, it will already be at or above operating temperature and will need a cooling period before an attempted restart.
A measure of heating of the motor that is appropriate is the line current supplied to the motor. It is possible to adapt a single controller to a wide range of motor currents by making an appropriate choice of current transformers and scalers in sensing the current in each phase of the motor power supply. It is evident that such a controller could also be operated by placing a single current transformer in one motor lead and using the signal from that current transformer to supply an input to a motor controller. However, it is usually desirable to place a current transformer in each motor lead and to combine signals from the current transformers to produce a single input to the controller. The use of multiple current transformers gives the additional ability to detect loss of one or more phases of the power supplied to the motor or an imbalance in phase currents due to a phase anomaly in the motor. In either case, whether one current transformer or a plurality is used, the result is to isolate the controller electrically from motor voltage. It is therefore appropriate to take the output of a motor controller to a contactor or other switch that will act in conjunction with other elements of a circuit to control power to the motor.